1. Field of the Invention
The present invention relates generally to the art of servovalves and servomechanisms, and more particularly to a novel method for providing air gaps of equal length in a torque motor assembly intended for use in a servovalve or servo-mechanism.
2. Description of the Prior Art
Servovalves and servomechanisms employing torque motors of the type herein illustrated and described are representatively shown in U.S. Pat. Nos. 3,023,782, 3,228,423 and 3,257,911, the aggregate disclosures of which are hereby incorporated by reference.
In such torque motors, the armature is conventionally supported by a flexure spring or flexure tube member such that, with the permanent magnets discharged, ideally, the armature plate portion is precisely centered between the two spaced pole-pieces. The torque motor is made operable by charging the permanent magnets, which results in one polepiece having a magnetic North, and the other, a magnetic South. Magnetic flux then flows from the North polepiece, through one air gap to the armature plate portion, through the armature plate portion, and then from the armature plate portion across the second air gap to the South polepiece. One or more such pairs of such active air gaps may be utilized with the armature of any specific torque motor design.
The armature is normally attracted to each polepiece by a force related to the magnitude of the flux which, in turn, is related to the length of the air gap in the flux-carrying direction. If the individual air gaps of each pair of air gaps are precisely equal in length, then no net force will be developed on the armature. If, however, one air gap is shorter than the other opposite air gap, then the armature will be attracted to the polepiece of the shorter gap.
The torque motor coils are located about the armature, and are energized with a d.c. electrical current to provide a command into the servovalve. This d.c. current will magnetically polarize the armature, one end being North and the other end being South depending upon the direction of the d.c. current. Additional flux induced by the coil current will flow through diagonally opposite air gaps of the torque motor and thereby create a corresponding torque on the armature.
It should be recognized that, in practice, many different types and configurations of torque motors may be used as the electrical input transducer of a servovalve. The symmetrical, double-ended armature type of torque motor having two pairs of air gaps, as illustrated herein, is merely representative of one species of such torque motors. The schematic for this type of torque motor is given is FIG. 11.26 on page 337 of the book "Fluid Power Control", Blackburn, Reethof and Shearer, published jointly by The Technology Press of MIT and John Wiley and Sons, Inc. (1960), the disclosure of which is hereby incorporated by reference. Numerous other schematics included in Section 11.2 of Chapter 11 of this book illustrate other torque motor configurations for developing both rotational and translational motion, all of which would benefit from the air gap fabrication technique herein disclosed.
If the air gaps are of unequal length in a typical electrohydraulic servovalve application, the servovalve may have a null offset such that other than a zero electrical input current must be impressed upon the servovalve coils to achieve zero hydraulic output.
Other delitescent results can occur from the presence of air gaps of unequal length in the torque motor of a servovalve, such as excessive temperature null shift and latching of the armature to a polepiece, such phenomena being well known and understood by those skilled in the art.
The individual air gap lengths (i.e., the distance between the armature and each polepiece) will typically range from 0.010 to 0.050 inch for torque motors of various physical sizes. Generally, the servovalve manufacturer attempts to maintain the desired air gap length within .+-. 0.0005 inch. This close tolerance cannot practically be obtained by accurate fabrication of the torque motor, as the dimensional tolerances of many individual parts contribute to determining the final air gap lengths.
Heretofore, the typical manufacturing procedure has been to assemble the servovalve torque motor (including the armature, flexure support, coils, permanent magnets, and polepieces) to the base mounting structure on a trial basis. The resulting air gaps are then measured by mechanical or optical means. The desired thicknesses of correcting shims could then be calculated. The torque motor is then disassembled and reassembled to incorporate the correcting shims of the calculated thickness. The air gaps are then remeasured and the torque motor assembly is either accepted or rejected for yet another trial with shims of different thicknesses.
An allowable degree of mismatch of air gap lengths is established for each servovalve design reflecting the desired level of quality and performance. Clearly, a manufacturing process which insures precisely matched air gaps without the tedious trail and error selection of shims, or other means, would yield high-quality servovalves in an expeditious manner.